drop incoming http requests to URI if external IP

With regards to the class match and matching a datagroup can i instead just define the addresses in the i-rule itself

You can, but your evaluation will get a little complex with more than two IPs/subnets. Example:

if { ( [IP::addr [IP::client_addr] equals 10.0.0.0/8] ) or ( [IP::addr [IP::client_addr] equals 172.16.0.0/12] ) or ( [IP::addr [IP::client_addr] equals 192.168.0.0/16] ) }

where [IP::addr ] is the Boolean operator you would to evaluate IP addresses in an if clause.